Adjacent channel interference, due to what is known as a “near-far” situation, has always been a problem in the multi-carrier deployment of CDMA mobile systems. This near-far problem manifests itself at the border of an overlay area where additional CDMA carrier is deployed for capacity or other reasons. As a mobile terminal that is communicating with a border base station in the overlay area and operating at a carrier frequency deployed in that area moves across the border of the overlay area into the underlay area that is covered by base stations operating at other carrier frequencies, the mobile terminal is supposed to switch frequencies and be handed off to a second base station in the underlay area for continued communication at one of these other carrier frequencies, usually called a “hard” handoff. If, however, the mobile terminal doesn't switch on time, it is possible that it may come close to a base station in the underlay area, while communicating with and being power-controlled by the base station in the overlay area, thus continuing to operate at that overlay base station's carrier frequency. When that carrier frequency is adjacent to a carrier frequency used by the base station in the underlay area that it is now near, the power of the uplink signal transmitted by the mobile terminal may be sufficiently large to interfere with that base station's reverse link on the adjacent carrier. Specifically, due to the inability of the base station in the underlay area receiver to totally suppress adjacent channel signals, the magnitude of sideband of the uplink signal transmitted by the mobile terminal that is within an operating band of such an adjacent frequency carrier used by the base station in the underlay area, in a worst case scenario, can result in the total shut-down of that carrier by the underlay base station, causing that underlay base station to drop all ongoing calls and preventing customers from initiating and receiving new calls on that carrier. Alternatively, but deleterious to service, the generated interference can shrink the second base station's coverage area and preclude that base station from providing service to mobile terminals in its coverage area.
The above-described near-far situation is likely to be a problem in CDMA systems when an additional carrier at a carrier frequency Fn is deployed as an overlay to alleviate capacity problems in underlay second or third generation (2G/3G) mobile systems. Various techniques have been incorporated on the network side to prevent the described near-far situation. These techniques generally utilize some signal quality or distance information gathered by the mobile terminal on carrier frequency Fn to ensure that it redirected and handed off to a neighboring underlay base station in the underlay area (with carriers F1, F2, . . . , Fn−1) before it can start interfering with the reverse link of any adjacent carrier used by any base station in the underlay area. A general problem with these prior art solutions is that redirect and handoff was often triggered before the underlay base station to which the mobile terminal is being redirected could reliably handle the call, causing frequent dropped calls. A Mobile Assisted Hard Handoff (MAHHO) mechanism defined by the IS-95B version of the CDMA standard has been implemented in 2G and 3G mobile terminals to alleviate this situation. In accordance with this mechanism, a mobile terminal, normally while in a border overlay base station's coverage area and when instructed to do so by the network, jumps to an underlay carrier, measures signal conditions coming from the base station transmitting at the underlay carrier, such as the mobile receive power at the underlay carrier frequency and the signal-to-noise-like Ec/Io measurement of the pilot signal transmitted by the underlay base station, returns to the overlay carrier, and reports the measured signal conditions back to the overlay base station. In response to these measurements, the base station can issue a timely redirection order.
A near-far situation is also likely to arise when the overlay system is the newly developed 1xEV system normally expected to be deployed on top of a 3G-1X underlay system to provide high speed data service at up to 2.4 Mb/s in selected commercial areas. A hybrid 1xEV/3G-1X mobile terminal will take advantage of the high-speed data 1xEV service in the overlay area where it is deployed. It will operate as a 3G-1X mobile terminal for lower-speed data service outside the overlay area and for voice service within both inside and outside the overlay area. Unlike the 3G and 2G systems, which are network-centric systems that are downward compatible, the 1xEV system was conceived to function largely as an independent stand-alone overlay system. Specifically, the IS-856 1xEV standard does not support the redirection from 1xEV to 3G-1X while on a data call, nor does it provide any means to inform a mobile terminal about the underlay system when it is idle so that it can make an informed autonomous decision when to switch. The 1xEV standard does provide, however, an option for the network to instruct all the mobiles in a base station's coverage area to unconditionally abandon its 1xEV carrier and to find a specific 3G-1X carrier. This creates a transition zone between the overlay 1xEV system and the underlay 3G-1X system, thereby wasting the capacity and the coverage of those 1xEV base stations on the border between the overlay and underlay system. The 1xEV standard also supports an option to redirect individual mobile terminals to the underlay system, but there is lack of support for any MAHHO type of reporting about signal quality of the target 3G-1X carrier. There is no reliable way, however, of determining whether or not a hybrid mobile terminal operating at the border and communicating with the overlay 1xEV system is likely to cause interference with a 3G-1X base station in the underlay area. Thus, a timely, reasonably reliable and non-wasting mechanism does not exist for switching from the overlay 1xEV system to the underlay 3G-1X system before interference with the a border base station in the underlay system may occur. The potential for a deleterious impact on the underlay 3G-1X system therefore could seriously impact the deployment of a 1xEV system.